Method for analyzing liquid metal and device for use in this method

ABSTRACT

A method for analyzing the composition of a bath comprising liquid metal, in which a laser beam is directed onto a surface of the bath and in which at least part of the metal forms a sample which is analyzed. In the method, at least at the location where the laser beam meets the bath, impurities at the surface of the bath are removed by purging with a stream of a purge gas. A device for analyzing the composition of a bath comprising liquid metal.

CROSS-REFERENCE TO RELATED APPLICATION

This claims the benefit under 35 USC 119 of Netherlands patent application number 1029612, filed on 26 Jul. 2005, incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a method for analyzing the composition of a bath of liquid metal and to a device for use in this method.

BACKGROUND OF THE INVENTION

A method and device for analyzing the composition of molten steel are known from British Patent Application GB 2154315A, incorporated herein by reference in its entirety.

According to the method described, a probe which is provided with means for transporting a beam of laser light is positioned above a bath of liquid steel. That part of the steel which the laser light impinges on emits element-specific radiation. The radiation emitted is passed via an optical system, which is likewise accommodated in the probe, to a spectrometer for analysis. The composition of the steel can be derived from the results of the analysis. This method is also known as LIBS (Laser Induced Breakdown Spectroscopy).

On the side facing the bath, the probe is provided with a ceramic tube which protects the probe from being affected by liquid steel and by slag which is present on top of the liquid steel.

To prevent liquid steel from penetrating into the ceramic tube, a gas pressure is applied inside the tube by a stream of a gas which is supplied via an inlet port of the probe and is discharged along outlet ports of the probe with a limited passage.

In the context of the present description, the term laser light is to be understood as meaning any form of electromagnetic radiation which is generated by a laser. Furthermore, the term metal is also to be understood as encompassing an alloy which substantially comprises one or more metals and other metals or non-metals.

One problem of the known method is that the material which the laser beam impinges on at the surface of the bath, such as steel and alloying elements in the prior art, reacts with the gas or with impurities which are inevitably present in the gas, such as oxygen if the gas is an inert gas.

The impurities react with the material and form compounds which in turn, on account of the energy of the laser beam, at least in part generate radiation, and thereby distort the measurement of the composition of the bath.

Another problem is that compounds which have such a high melting point or boiling point that they do not melt or evaporate, but rather form a solid crust where the laser beam is supposed to meet the bath, may form. This problem plays a significant role in particular in the case of reactive metals.

If the composition of the bath fluctuates, the solid crust shields the bath from further analysis, with the result that it is not possible to measure the correct current composition of the bath.

The problems are significant in particular in the case of measurements which last for a prolonged period of time on account of the requirement for statistical accuracy.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for analyzing a metal, in which the measurement accuracy is not affected by impurities in the gas.

It is another object of the invention to provide a method for analyzing a metal which is also suitable for analyzing reactive metals.

Yet another object of the invention is to provide a method for analyzing a metal which is also suitable for measuring low concentrations of an element in a bath of a metal or for long-term measurements.

These and other objects are achieved by a method for analyzing the composition of a bath comprising liquid metal, in which a laser beam is directed onto a surface of the bath and in which at least part of the metal forms a sample which is analyzed, characterized in that, at least at the location where the laser beam meets the bath, impurities at the surface of the bath are removed by purging with a stream of a purge gas.

The stream of the purge gas entrains impurities in the form of undesirable reaction products which have been formed and carries them outside the region of the surface where the laser beam meets the bath. Tests have shown that removing impurities can be carried out so effectively that it is possible to carry out measurements continuously, so that even accurate measurement and determination of a changing composition of the bath is possible.

The method according to the invention is particularly advantageous if the liquid metal is a liquid aluminium alloy.

Aluminium has a particularly high affinity for oxygen, in such a manner that even the very low oxygen potential of 10⁻⁷ in argon, which is customarily used as purge gas, is too high to allow analysis of the composition of an aluminium bath, certainly if low concentrations of alloying elements or impurities need to be determined. Prior to the invention, therefore, LIBS was unsuitable for determining the composition of a bath of liquid aluminium continuously or with a high level of accuracy, as required for the production of aluminium alloys.

A further improvement to the method according to the invention is achieved in an embodiment which is characterized in that the velocity and direction of the stream of the purge gas are selected in such a manner that the surface of the bath adjacent to the location where the laser beam meets it has a convex meniscus.

It has been found that by selecting the direction in which the gas is fed to the surface and the quantity of gas, it is possible to promote the formation, maintenance and height of a convex meniscus. It is particularly effective to supply the purge gas in such a manner that the purge gas is discharged radially over the convex meniscus and thereby entrains impurities.

According to one particularly effective embodiment of the invention, a submerged pipe system provided with a top side, an underside and a casing is at least partially placed in the bath, and the stream of the purge gas is fed into the submerged pipe system above the bath and is directed onto the bath at least at the location where the laser beam meets the bath.

The submerged pipe system extending into the bath of liquid metal causes a convex meniscus of the molten metal inside the casing of the submerged pipe system. Solid or liquid impurities which form on the meniscus are discharged by the stream of purge gas to the region outside the laser beam. It has been found that, in the case of a flat top side of a bath of, for example, molten aluminium, a depression up to 10 mm deep can form at the location where the laser beam meets the surface of the bath. Dross collects in this depression, making reliable measurement impossible. This embodiment of the invention avoids this problem.

A further embodiment of the method according to the invention is characterized in that purge gas in the submerged pipe system is discharged from the submerged pipe system below the surface of the bath. For a selected diameter of the casing pipe, the depth to which the casing is submerged and the velocity of the stream of gas, it is in this way possible that the impurities are discharged out of the submerged pipe system.

The method according to the invention is of particular benefit if the liquid metal is a liquid aluminium alloy. Liquid aluminium is particularly reactive and forms a crust of aluminium oxide, which distorts the measurement, even in the presence of very small quantities of oxygen, originating from the purge gas or from leaks. With the method according to the invention, aluminium oxide which is formed is discharged by the purge gas to outside the region where the laser beam meets the aluminium surface.

The invention is also embodied by a device for use in the method for analyzing the composition of a bath comprising liquid metal, which device is provided with a submerged pipe system provided with a top side, an underside and a casing having an inner wall and an outer wall, a source for generating a beam of laser light and a gas supply tube for feeding purge gas into the submerged pipe system, characterized in that the gas supply tube is provided with a passage having a reduced bore size, in order to at least locally increase the flow velocity of the gas in the direction of the bath, and runs inside the casing in such a manner that during use the bath inside the casing forms a convex meniscus. It is preferable for the gas supply tube inside the casing to at least in part be concentric with the casing.

When the device is in use, the submerged pipe system is submerged in the bath of liquid metal, forming a convex meniscus. It has been found that the height of the convex meniscus can be increased by discharging the purge gas along the underside of the submerged pipe. It is preferable for the purge gas to be supplied with a selected velocity in the direction of the highest point of the convex meniscus. One additional advantage is that impurities are entrained along the meniscus by the purge gas.

It has been found that the momentum which the laser beam imparts to the molten metal causes drops of molten metal to jump out of the bath and be deposited on, inter alia, the optical system which is used to guide the laser beam to the bath and to guide any element-specific radiation to an analysis apparatus, or on a gas pipe for discharging the gas sample to an analysis apparatus.

This embodiment of the invention also prevents drops of liquid metal from reaching the optical system or a pipe for the gas sample, on the one hand on account of the reduced size of bore and on the other hand on account of the higher gas velocity.

A further improvement to the device according to the invention is achieved with an embodiment which is characterized in that the casing of the submerged pipe system is provided, in the part which in use extends into the bath, with outlet openings and/or with slots arranged in the underside of the casing.

Surprisingly, it has been found that in this embodiment the meniscus is self-stabilizing or is at least more self-stabilizing than without this measure, i.e. the meniscus has a symmetrical profile in the submerged pipe, in particular if the casing has a circular cross section.

Moreover, it has been found that oscillations which occur in this embodiment caused by the momentum of the laser beam are more successfully attenuated in an embodiment in which slots and/or openings run with an increasing cross section from the inside of the casing towards the outside thereof.

A further preferred embodiment of the method according to the invention is characterized in that at least part of the inner wall of the casing of the submerged pipe system is made from a material which is not wettable by the liquid metal.

This embodiment promotes the formation and height of a convex meniscus still further. Moreover, it has the effect of providing the submerged pipe with a longer service life on account of reduced chemical attack, since the casing does not come into contact with the material to the same extent.

The invention will be explained in more detail below on the basis of a diagrammatic drawing of a measurement arrangement suitable for carrying out the method according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a diagrammatic cross section through a measurement arrangement for measuring the composition of a bath according to the invention, and

FIG. 2 a shows a side view of the submerged pipe; in this case of the bottom pipe 3 in the form of a cylindrical pipe.

FIG. 2 b shows a view of the underside of the submerged pipe of FIG. 2 a, in particular it shows a view of the underside of the casing of a submerged pipe system for use in the method according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, 1 denotes part of a bath of molten metal, such as a bath of a molten aluminium alloy. A submerged pipe system 2, which in the embodiment shown is provided with a casing 3, extends into the bath 1. A gas supply tube 4 runs concentrically inside the casing 3. The casing 3 and the gas supply tube 4 are coupled to one another by means of O-rings 5, which also provide a gastight seal.

As a result of part of the casing 3 being submerged in the bath 1, a convex meniscus 6 is formed inside the casing 3. It is preferable for at least part of the inner wall of the casing 3 to be made from a non-wetting material. A laser beam 15, generated by a laser source (not shown), meets the meniscus at the location of the meeting plane 7, which is located above the focal point of the laser beam in order to prevent positioning problems and in order to prevent gas located above the meniscus from forming a plasma.

The gas supply tube 4 is provided with a tapered downstream (relative to the direction of gas flow) end portion defining a passage 8 with a central narrowed section 9 at a downstream end of the end portion. The central narrowed section 9 has a reduced bore size relative to the inside diameter (bore size) of an upstream end of passage 8.

The direction of the stream of the purge gas originating from a gas source (not shown in more detail) is denoted by arrows 10, 11 and 12.

As a result of the reduced size of bore, at the downstream discharge end of the central narrowed section 9, the purge gas acquires an increased velocity as it flows through the bore. The stream of purge gas flows along the meniscus and entrains solid and possibly liquid particles in the direction indicated by arrow 13, so that it is always a clean meeting plane 7 which is exposed to the laser beam. Moreover, the gas stream promotes the maintenance and height of the convex meniscus. The process of keeping the meeting plane 7 clean is further promoted by the downward curvature of the meniscus.

The purge gas leaves the submerged pipe in the form of gas bubbles 14 and entrains the solid particles.

FIG. 2 a shows a side view of the submerged pipe; in this case of the bottom pipe (casing) 3 in the form of a cylindrical pipe.

FIG. 2 b shows a view of the underside of the submerged pipe (casing) 3.

The wall 20 of the casing 3 is provided with toothed slots 21. It is preferable for the cross section of the slots to increase from the inside outwards, as shown in FIG. 2 b. The result of this is that the flow velocity of the gas on the inside of the submerged pipe is greater than on the outside, which has a stabilizing effect on the position of the meniscus and also ensures that the metal in the submerged pipe continues to move and is therefore a good reflection of the metal in the molten bath. In addition or as an alternative, the casing may be provided with holes 22 which are submerged in the bath when using the method. The slots 21 and holes 22 have a stabilizing and homogenizing effect on the shape of the meniscus. Moreover, it is assumed that they cause an additional flow around the underside of the submerged pipe (casing) 3, which contributes to the liquid metal in the submerged pipe (casing) 3 being constantly refreshed and therefore being a good reflection of the molten metal in the remainder of the bath 1. 

1. A method for analyzing the composition of a bath comprising liquid metal, in which a laser beam is directed onto a surface of the bath and in which at least part of the metal forms a sample which is analyzed, comprising, at least at the location where the laser beam meets the bath, removing impurities at the surface of the bath by purging with a stream of a purge gas.
 2. The method according to claim 1, wherein the velocity and direction of the stream of the purge gas are selected in such a manner that the surface of the bath adjacent to the location where the laser beam meets it has a convex meniscus.
 3. The method according to claim 1, wherein a submerged pipe system provided with a top side, an underside and a casing is at least partially placed in the bath, and the stream of the purge gas is fed into the submerged pipe system above the bath and is directed onto the bath at least at the location where the laser beam meets the bath.
 4. The method according to claim 3, wherein purge gas in the submerged pipe system is discharged from the submerged pipe system below the surface of the bath.
 5. The method according to claim 1, wherein the liquid metal is a liquid aluminium alloy.
 6. A device for analyzing the composition of a bath comprising liquid metal, comprising: a submerged pipe system provided with a top side, an underside and a casing having an inner wall and an outer wall, a source for generating a beam of laser light, and a gas supply tube for feeding purge gas into the submerged pipe system, the gas supply tube for feeding the purge gas in the direction of the bath, and to run inside the casing, in such a manner that during use the bath inside the casing forms a convex meniscus along which purge gas flows.
 7. The device according to claim 6, wherein the gas supply tube has a reduced bore size for at least locally increasing the flow velocity of the purge gas.
 8. The device according to claim 6, wherein the casing of the submerged pipe system is provided, in the part which in use extends into the bath, with outlet openings and/or with slots arranged in the underside of the casing.
 9. The device according to claim 8, wherein the slots and/or openings run with an increasing cross section from the inside of the casing towards the outside thereof.
 9. The device according to claim 6, wherein at least part of the inner wall of the casing of the submerged pipe system is made from a material which is not wettable by the liquid metal.
 10. The device according to claim 6, further comprising: the liquid metal bath; wherein the casing has a downstream end submerged into the bath; a portion of the bath being within the downstream end of the casing and having a convex meniscus; the gas supply tube has a downstream end concentric with the casing and the gas supply tube passage is located within the casing for discharging purge gas into the casing onto the portion of the bath within the downstream end of the casing; the source for generating a beam of laser light and for discharging the beam through the supply tube passage onto the portion of the bath being within the downstream end of the casing; a downstream end of the gas supply tube passage having the bore size for at least locally providing the purge gas in the direction of the bath with sufficient flow velocity in the casing to form the convex meniscus along which purge gas flows on the bath inside the casing. 